Switched reluctance motors (SRMs) have a wide variety of applications. Low-power versions are used in washers and dryers. Medium-power variants are found in applications such as industrial general-purpose drives and train air-conditioning drives. Fans, pumps and wind power generation are some of the applications for high-power SRMs. High-speed versions of SRMs are used in applications such as centrifuges for medical applications and aircraft jet engine starter motor generators. SRMs are also used in linear drive applications including low-speed direct propulsion drive transit systems.
One prior converter for controlling SRMs is an asymmetric half-bridge converter. A typical asymmetric half-bridge converter circuit is shown in FIG. 1 and comprises two discrete switching components and two diodes per phase. The phase winding is connected in series between the switching components. The unconnected ends of the switching components are connectable to a respective pole of a power supply. One end of each diode is connectable in reverse bias to a respective pole of the power supply. The other end of each diode is connected between the phase winding and the switching component connectable to the other pole of the power supply.
The asymmetric half-bridge converter can be operated in three modes: a charging mode, a freewheeling mode, and a discharging mode. In the charging mode, both switching components are closed, thereby allowing the power supply to charge the phase winding. In the freewheeling mode, one switching component is open and the other is closed, the current thereby circulating, or freewheeling, through the closed switching component and diode connected in parallel to the closed switching component. In the discharging mode, both switching components are open, the current thereby discharging from the phase winding through both diodes back to the power supply and recharging the power supply.
A disadvantage of the asymmetric half-bridge converter, however, is the high number of switching components required, that is, two discrete switching components per phase. This increases the cost of the converter. This also makes the circuitry more complex and increases the number of components that need to be handled during manufacturing. This increases manufacturing labour and time, which in turn, further increases cost.
One variation of the asymmetric half-bridge converter circuit described above utilizes readily available switch modules, with each discrete switching component in the circuit replaced by a respective switch module. That is to say, two switch modules are required per phase. Each switch module is a pre-manufactured integrated unit, itself including two switching components interconnected in series, with each switching component comprising a transistor and a diode connected in reverse bias in parallel across the transistor. Thus, when used in the asymmetric half-bridge converter circuit described above, one switching component in each switch module is redundant.
Although this variation utilizes readily available switch modules, possibly saving manufacturing time, the use of these switch modules results in large sized, heavier, and more costly converters.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.